OK here is my UK based scenario which might help clarify why short- term untrained volunteers can be unsuitable to work with vulnerable people, despite good intentions. This is a real-life-I-was-there situation. (One of many, many like it)
Family A are happy and functional: 2 parents, 2 teenagers doing well in school, polite, helpful kids etc. etc.
One year they decide as a family that they are going to give up their Christmas day and go and work together at the local homeless soup kitchen.
The mother goes to meet with the coordinator and asks to put their names down.
She is told that this will not be possible.
Why? don't they need extra hands on deck- peeling the potatoes and washing up?
Of course they do- they are always short of hands but this particular project is for super vulnerable homeless women and Christmas is an extremely difficult time for them. They have nearly all had their children removed into the care system and their emotions are very close to the surface. On Christmas Day they need people around them who they trust, who can talk them down if they get desperate and who they can break down with should they need to.
It is not a good time for them to be seeing unfamiliar faces and also it is a time where the possibility of suicide, self harm and violence is high.
The mother is told that if she wants to help she can come weekly to used clothes drop off and take a pile of washing to take home and clean and fold.
The mother declines and is pretty cross. She asks if there is anywhere else the family can help out on Christmas Day, as this is what they want to do.
Volunteering is NOT bad. Sloppy volunteer placement IS bad. Gate keeping of vulnerable people despite volunteers good intentions IS sometimes necessary to protect everyone concerned. Access to project beneficiaries should not be give just because volunteers want it to be- and expertise is valuable and should be listened to!